Vitamin A metabolism gives rise to several active forms of retinoic acid (RA) which are involved in regulating gene expression during development, regeneration, and in the growth and differentiation of epithelial tissues [Maden, 1992; Chambon, 1995; Mangelsdorf, 1995; Gudas, 1994; Lotan, 1995; Morriss-Kay, 1996]; has been linked apoptosis, or programmed cell death in a number of cell types; and to have anticarcinogenic and antitumoral properties [Lotan, 1996].
Early studies of retinol deficiency indicated a correlation between vitamin A depletion and a higher incidence of cancer and increased susceptability to chemical carcinogenesis [Chytil, 1984]. Several animal models have been used to demonstrate the effectiveness of retinoids in supressing carcinogenesis in a variety of tissues including skin, mammary epithelia, oral cavity, aerodigestive tract, liver, bladder and prostate [Moon, 1994]. These studies have led to the preventative use of retinoids to treat premalignant lesions including actinic keratosis and oral leukoplakia, as well as in the prevention of secondary tumours of the head and neck and the recurrence of non-small cell lung carcinomas, and basal cell carcinomas [Hong, 1994; Lippman, 1995]. RA itself has been found to be useful therapeutically, notably in the treatment of cancers, including acute promyelocytic leukemia (APL), tumors of the head and neck, and skin cancer, as well as in the treatment of skin disorders such as the premalignancy associated actinic keratoses, acne, psoriasis and ichthyosis. There is evidence that the effectiveness of RA as an anti-tumour agent is at least partially due to induction of cellular differentiation and/or inhibition of proliferation [Lotan, 1996]. Studies over the past several years indicate that a high proportion of patients with acute promyelocytic leukemia (APL) achieve complete remission after a short period of treatment with all-trans RA. Unfortunately, this high rate of remission is in most cases brief. Following relapse, patients are clinically resistant to further treatment with RA [Warrell, 1994; Warrell, et al., 1994; Chomienne, 1996; Muindi, 1992]. The nature of this resistance is unknown. Interestingly, leukemic cells taken from patents exhibiting clinical resistance to RA have been shown to be sensitive to the differentiating action of RA when grown in vitro (Muindi, 1992; Muindi, 1994). This suggests that pharmacokinetic mechanisms may account for the acquired resistance to RA. This possibility is supported by studies showing that peak plasma concentrations of RA were much higher in patients after initial administration than in patents treated following relapse. This decrease in peak plasma RA concentration was accompanied by a 10-fold increase in urinary 4-oxo-retinoic acid concentration. In addition, ketoconazole, a broad spectrum inhibitor of cytochrome P450 function was shown to modulate RA pharmacokinetics in vivo [Muindi, 1992; Muindi, 1994]. It is therefore likely that RA increases the rate of its own metabolism which in turn results in the inability to sustain effective therapeutic doses of RA. Therapeutic administration of RA can result in a variety of undesirable side effects and it is therefore important to establish and maintain the minimal requisite doses of RA in treatment. For example, RA treatments during pregnancy can lead to severe teratogenic effects on the fetus. Adverse reactions to RA treatment also include headache, nausea, chelitis, facial dermatitis, conjunctivitis, and dryness of nasal mucosa. Prolonged exposure to RA can cause major elevations in serum triglycerides and can lead to severe abnormalities of liver function, including hepatomegaly, cirrhosis and portal hypertension.
RA metabolism may also account for the lack of response of certain tumors to RA treatment. For example, recent studies have shown that cytochrome P450 inhibitors that block RA metabolism, resulting in increased tissue levels of RA, may be useful therapeutic agents in the treatment of prostate cancer [Wouters, 1992; De Coster, 1996]. Thus RA metabolizing cytochrome P450s may be useful targets for the treatment of a number of different types of cancer.
The classical view of vitamin A metabolism holds that all trans-RA, the most active metabolite is derived from conversion of retinol to retinaldehyde to RA through two oxidation steps and that RA is further metabolized to the polar derivatives 4-OH RA and 4-oxo RA [Blaner, 1994; Napoli, 1995; Formelli, 1996; Napoli, 1996]. It is unknown whether the 4-oxo- and 4-OH- metabolites are simply intermediates in the RA catabolic pathway or whether they can also have specific activities which differ from those of all-trans RA and 9-cis RA. Pijnappel et al. [Pijnappel, 1993] have shown that, in Xenopus, 4-oxo-RA can efficiently modulate positional specification in early embryos and exhibits a more potent ability to regulate Hoxb-9 and Hoxb-4 gene expression than all-trans RA. 4-oxo-RA has been found to bind to retinoic acid receptor-β (RAR-β) with affinity comparable to all-trans RA [Pijnappel, 1993] but poorly to RAR-γ [Reddy, 1992], suggesting that this metabolite exhibits some receptor selectivity. 4-oxo-RA also binds to cellular retinoic acid binding protein (CRABP) but with an affinity slightly lower than that of all-trans RA [Fiorella, 1993]. Takatsuka et al. [Takatsuka, 1996] have shown that growth inhibitory effects of RA correlate with RA metabolic activity but it is unknown whether there is a causal relationship between production of RA metabolites and growth inhibition. The asymmetric distribution of these metabolites in developing embryos suggests that they may be preferentially sequestered or generated by tissue specific isomerases [Creech Kraft, 1994]. The normal balance of these metabolites is dependent upon rate of formation from metabolic precursors, retinol and retinaldehyde [Leo, 1989], and rate of catabolism. Little is presently known about the enzymes involved in this metabolic scheme, in particular the catabolism of RA.
The catabolism of RA is thought to be initiated by hydroxylation either at the C4-, or C18-position of the β-ionone ring of RA [Napoli, 1996]. The C4- hydroxylation step is mediated by cytochrome P450 activity, as judged by the ability of broad spectrum P450 inhibitors such as ketocoazole and liarazole to block 4-hydroxylation [Williams, 1987, Van Wauwe, 1988; Van Wauwe, 1990, Van Wauwe, 1992, Wouters, 1992]. In certain tissues, including testis, skin and lung and in numerous cell lines, such as NIH3T3 fibroblasts, HL 60 myelomonocytic leukemic cells, F9 and P19 murine embryonal carcinoma cell lines and MCF7, RA metabolism can be induced by RA pretreatment [Frolik, 1979, Roberts, 1979a and b; Duel, 1992; Wouters, 1992]. Studies involving targetted disruption of RAR genes in F9 cells suggest that RARE and RAR-γ isoforms may play a role in regulating the enzymes responsible for this increased metabolism [Boylan, 1995].
The glucuronidation of RA is a significant metabolic step in the inactivation of RA [Blaner, 1994; Formelli, 1996]. The elimination of RA may require oxidation to 4-oxo, followed by conjugation to form the 4-oxo all-trans RA glucuronide. This is supported by studies in both primates and humans showing that the 4-oxo RA glucuronide is the only retinoid conjugate found in urine [Muindi, 1992; Muindi, 1994]. The fact that following RA therapy, 4-oxo RA is not present or barely detectable in serum, suggests that oxidation may be the rate limiting step in this process.
It has recently been shown that 4-oxoretinol (4-oxo-ROL) can have greater biological activity than retinol. The 4-oxo-ROL is inducible by RA in F9 and P19 mouse teratocarcinoma cells [Blumberg et al., 1995; Achkar et al., 1996].
It is known that zebrafish fins regenerate through an RA sensitive process which utilizes many gene regulatory pathways involved in early vertebrate development [White, 1994; Akimenko, 1995a & b].
As far as the inventors are aware, cytochrome P450s involved in the metabolism of RA in extrahepatic tissues remain uncharacterized at the molecular level.